In a conventional reproduction system, when reproducing a digital audio signal, the digital audio signal is converted into an analog audio signal, amplified to a variable output level, and is then transmitted to the speaker. However, with the dissemination of digital audio technology, in recent years, techniques for driving a speaker without converting a digital audio signal into an analog signal have developed.
For example, one of the techniques uses a so-called pulse-density modulated one-bit binary digital signal which is produced by a high-speed sampling technique such as .DELTA..SIGMA. modulation. In this technique, according to a binary number (1 or 0) of the one-bit digital signal, the flow of a driving current to a speaker is started or stopped, or the flowing direction of the driving current is switched.
The frequency that a binary signal has a value or 1 varies depending on the amplitude of an original audio signal. This feature is discussed in detail below with reference to FIG. 10.
a) When the level of the original audio signal exceeds zero, the binary signal tends to have a value 1. PA1 b) When the level of the original audio signal is zero, the binary signal has a value 0 at the same frequency as it has a value 1. PA1 c) When the level of the original audio signal is below zero, the binary signal tends to have a value 0.
When reproducing an analog audio signal whose signal level exceeds zero and falls below zero, if the speaker driving device is arranged to stop the flow of a driving current when the binary signal is 0 and to start flow of the driving current when the binary signal is 1, only a driving force which can drive the speaker in one direction is generated, resulting in poor driving efficiency. To overcome such a drawback, the speaker must be driven in directions corresponding to the positive and negative levels of the original analog audio signal by causing a driving current to have a bias.
In the meantime, the speaker driving device which drives a speaker by switching the flowing direction of a driving current is capable of driving the speaker in a plurality of directions. However, to start the flow of a driving current to a conventional voice coil by switching its flowing direction, a complicated switching circuit is required.
Moreover, in a system where a power for driving a speaker is produced based on a one-bit digital audio signal, when a voltage to be applied to the voice coil fluctuates and includes noise components, the fluctuation and noise components directly affects the speaker driving force. This may cause distortion and noise in the reproduced sound.
Additionally, with the speaker driving device which stops or starts the flow of a driving current according to a binary signal, when the level of the original analog signal is 0, the possibility that the driving current flows to a speaker is about 50% due to the feature of the binary signal as stated in b), resulting in poor power efficiency.
Similarly, the speaker driving device which drives a speaker by switching the flowing direction of a driving current has the following problems due to the feature of a binary signal. For example, if the driving current is arranged to flow in a specific direction when the binary signal has a value 1 and to flow in the opposite direction when the binary signal has a value 0, the driving current always flows in one of the two directions to the speaker when the level of the original analog signal is 0. This arrangement also significantly lowers the power efficiency.